Presently, in computers or information processing devices, data is processed by moving electrons through copper wires; however, recently, photonics technology in which data is transmitted, processed, or computed using a flow of photons is being studied vigorously.
Thanks to the development of photonics technology, several new communication devices are being developed, and optical communication technology for transmitting data using light is being commercialized for general consumers to enable use of optical communication networks having a high bandwidth. Consumers can connect personal computers, servers, and other electronic appliances to an optical communication network at low cost.
An example of photonics technology is providing a waveguide on a silicon chip, and dividing the waveguide into two to separate a single optical beam into two, and then applying charges to one of the separated optical beams to induce a phase transition. When the separated optical beams are put together, the phase transition between the two optical beams is detected and thus an on/off signal at a speed of 1 GHz, that is, a speed at which data of a billion bits per second or greater can be transmitted, can be detected. By combining such photonic circuits, a logic circuit or a frequency filter can be realized. The operation speed of the photonic circuit is approximately 50 times greater than in the conventional art. Using silicon photonics technology, applications requiring higher bandwidth such as faster internet, high performance computers, super high definition displays, and image processing systems can be developed.
The reason that fiber optics is applied to silicon chips has to do with bandwidth considerations, as described above. A bandwidth of 1 GHz means that one billion bits of information can be processed per second with respect to a single optical fiber. The amount of information that will be transferred through networks is expected to increase by more than 10 times in the future. Also, even when it is a single photonic link, when multiple data channels transmitting a plurality of light having different wavelengths (or different colors) are formed, a great amount of data can be transmitted at the same time, maintaining the 1 GHz bandwidth.
An optical fiber cable is not affected by electromagnetic noise, which has been a problem in existing copper connections, has no danger of entanglement of wires, and does not generate heat either. In the case of copper wires, electrons can move a maximum of 50 cm at a speed of 10 Bbps (B=billion) due to impurities inside the copper and the irregularity of the molecular structure of copper. However, in the case of optical fiber, photons instead of electrons pass through a free space or a medium corresponding to a free space (e.g., a waveguide), and thus can move more than 10 km at a speed of maximum 150 Tbps (T=trillion). When using wavelength division multiplexing, one optical fiber can carry 40 channels, and thus can theoretically have a speed of 6 Pbps (P=Peta).
Various devices, in which a photonic circuit is integrated on a silicon chip as a super high density integrated circuit, have been developed. Examples of such devices are a frequency filter such as a notch type filter using a ring resonator, digital filters such as a FIR filter, an IR filter, etc., and logic circuits. As another example, not only the silicon chip but also copper wires of a printed circuit board are replaced with an optical fiber or optical waveguide. However, a memory device has not been developed yet.
In the case of a central processing unit (CPU), a plurality of memory devices such as a calculation processing unit performing calculations at high speed using a combination of logic circuits, a register storing the result of calculation temporarily, and a level cache memory (L2 cache) for efficient calculation are needed. However, an example of a device using silicon photonics that can be used as a memory device is only a device in which a flip-flop is formed by combining a NAND or NOR logic circuit to store 1 bit of information. A memory device that is realized by combining photonic logic circuits has a significantly large size, and thus it is very inefficient in respect of integration to manufacture a memory device storing data of only 1 bit by combining a plurality of logic circuits.
That is, when photonics technology is applied substantially, logic circuits or frequency filters can be realized as photonics circuits; however, memory devices such as register or L2 cache memory cannot be realized directly. For example, a calculation processing unit can realize a logic circuit using a photonic circuit; however, a memory unit cannot directly use a photonic circuit due to the spatial limit and thus has to use conventional DRAM or SRAM.
FIG. 1 is a plane view illustrating a structure of a conventional DRAM. A DRAM 10 includes cells distinguished by a bit line 11, and a word line 12, and a sense amp transistor 13 and a capacitor 14 included in each of the cells. Each cell stores data of 1 bit or more, wherein the amount of data depends on the structure of a chip. Each cell has a distinguished address according to the line and row, and memory is accessed using addresses. An end portion of the sense amp transistor 13 is connected to the bit line 11, and a gate of the sense amp transistor 13 is connected to the word line 12 to perform writing, reading, and refreshing of the DRAM memory 10. The capacitor 14 of each cell is discharged after one access period is terminated. The sense amp transistor 13 receives power from the bit line 11 before one access period is terminated, and refreshes the data of the DRAM 10 by charging the capacitor 14.